Optical waveguide modulators used in high-speed optical communications, such as those based on waveguide Mach-Zehnder (MZ) interferometric structures, may require active control of their operating conditions, and in particular of their bias voltage that sets the relative phase of interfering light waves in the modulator in the absence of the modulation signal. The waveguides of the modulator are typically formed in an electro-optic material, for example a suitable semiconductor or LiNbOx, where optical properties of the waveguide may be controlled by applying a voltage. Such a waveguide modulator may be a part of an optical integrated circuit (PIC) implemented in an opto-electronic chip.
Very high speed optical systems may benefit from Quadrature Amplitude Modulation (QAM), which may be realized using a quadrature modulator (QM) that may be implemented using nested MZ interferometric structures. Such structures typically require controlling several bias voltages. For example, a QAM optical signal may be generated by splitting light from a suitable light source between two MZ modulators (MZM) driven by an in-phase (I) and a quadrature (Q) complements of an electrical QAM signal carrying data, and then combining the resulting I and Q modulated light signals in quadrature, i.e., with a 90°, or π/2 radians (rad), relative phase shift ϕIQ. For example the two MZMs of such QM may each be modulated by a BPSK (binary phase shift keying) signal while being biased at their respective null transmission points for push-pull modulation. When their outputs are added together in quadrature, i.e. with the relative phase shift ϕIQ=π/2, a QPSK signal (Quaternary phase shift keying) results. While the bias voltages of the two MZMz for the push-pull modulations may be controlled by monitoring the time-averaged optical power at the output of the modulator, the output averaged optical power is insensitive to the IQ phase shift ϕIQ, so that a drift of the bias voltage VIQ away from a bias point needed to maintain the desired IQ phase shift may be more difficult to monitor and correct for. Known techniques for monitoring the IQ phase shift ϕIQ in the modulator typically require high-bandwidth processing of the control signal, which is difficult to implement in practice.
Furthermore, existing feedback schemes that are used to control a set point of an optical MZ modulator typically require tapping off a small portion of the modulator output power to analyze for bias drifts. The tapped-off portion of the output power, although relatively small, should still be large enough in the conventional bias control techniques so that relatively small bias drifts may still be detected, which may measurably reduce the useful optical power from the modulator.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for controlling a bias point of an optical waveguide modulator suitable for use in high-speed optical systems.